1. Field of the Invention
The present invention relates to a liquid crystal display apparatus and to an alternating current driving method therefore and, more particularly, to improvement of an alternating current driving type liquid crystal display apparatus adapted to every frame is reverse the polarity of a signal voltage for a switching device, such as a thin film transistor.
2. Description of the Related Art
It is known that a phenomenon, in which characteristics of a liquid crystal is deteriorated, that is, what is called image sticking occurs in a case where a signal voltage of the same polarity is applied to the same pixel electrode for a long time. Hitherto, to prevent an occurrence of such image sticking, a technique called an “alternating current driving” method, according to which the polarity of a signal voltage written to each pixel is reversed, has been employed. For example, an occurrence of the image sticking is prevented by reversing the polarity of a signal voltage to be applied to the same pixel every frame of a video signal (see, for example, JP-A-11-149277).
FIG. 8 is a view showing the configuration of a conventional liquid crystal display apparatus. This liquid crystal display apparatus 100 is called an active matrix typed display apparatus, and includes a liquid crystal display panel portion 2, a source driving portion 101, and a gate driving portion 4. In the liquid crystal display panel portion 2, many source lines 6 and many gate lines 7, which are intersected with the source lines 6. A pixel having a switching device constituted by a thin film transistor is formed at each of the intersections there between.
The source driving portion 101 includes a signal processing portion 8, source driver ICs 9 and a polarity control portion 102, and supplies a signal voltage to each pixel through the source line 6 according to a video signal. The gate driving portion 4 includes a gate control portion 103 and gate driver ICs 104, and enables or disables the gate in the thin film transistor of each of pixels, which are respectively associated with the gate lines 7, through the associated gate line 7.
The signal processing portion 8 determines a signal voltage, which is associated with each of the pixels, according to the amplitude level of the video signal and sequentially outputs voltage data, which represent such voltages, to the source driver ICs 9. The polarity control portion 102 generates a polarity reversal signal 105, which is used for reversing the polarity of the signal voltage, according to a horizontal synchronization signal and a vertical synchronization signal, which are extracted from the video signal, and outputs the generated polarity reversal signal to each of the source driver ICs 9. Each of the source driver ICs 9 serially applies signal voltages to the source lines 6 according to this polarity reversal signal 105 and the voltage data to thereby drive each of the thin film transistors.
The gate control portion 103 outputs control data to the gate driver IC 104 according to the horizontal synchronization signal and the vertical synchronization signal, which are extracted from the video signal, to thereby control the enabling and the disabling of the gate associated with each of the gate lines 7. The gate driver IC 104 sequentially enables and disables the gates according to this control data through the gate lines 7. That is, the gates respectively associated with the gate lines 7 are sequentially enabled, so that only each of the thin film transistors provided on the single gate line 7, the associated gate of which is in an enabled state, is writable. That is, signal voltages are sequentially written to the thin film transistors, which are in such a state, through the source lines 6. At that time, the polarity of the signal voltage applied to each of the thin film transistors is reversed in response to a polarity reversal signal 105 every vertical scanning period. That is, the positive or negative polarity of the voltage applied to each of the pixels is reversed every frame of the video signal.
Further, in a dot inversion driving type apparatus, a control operation of reversing each of the polarities of the signal voltages respectively associated with adjacent pixels is performed. That is, each of the polarities of signal voltages respectively applied to the adjacent pixels provided on the gate line 7 is reversed. Additionally, each of the polarities of the signal voltages respectively applied to adjacent pixels provided between the gate lines 7 is also reversed. Such an alternating current driving operation can effectively prevent the pixels from being baked.
Generally, in a case where an interlace signal is inputted to a liquid crystal display apparatus, it is necessary to perform deinterlacing (that is, format conversion of an interlace signal to a progressive signal) by scanning-line interpolation. Usually, an interlaced scanning (or interlacing) technique to be used for enhancing resolution by utilizing an amount of information, which is as small as possible, and for smoothing motions is performed on images taken by a video camera and those received by a television receiver. One frame of such a video signal (that is, an interlace signal) is divided into two fields, and scanning is performed two times respectively associated with the two fields. For example, in the case of NTSC signals, each frame, whose frame period is ( 1/30) seconds, thereof is divided into an odd-numbered field and an even-numbered field. In a first half (( 1/60) seconds) of the frame period, only odd-numbered scanning lines are shown. Then, in the next half (( 1/60) seconds) of the frame period, only even-numbered scanning lines are shown.
However, the aforementioned conventional liquid crystal display apparatus has a problem in that when showing a noninterlace signal obtained by performing scanning-line interpolation on an interlace signal, a pixel, on which an alternating current driving operation cannot be performed, appears in a case where a same image is continuously shown over plural frames.
FIG. 9 is a state view illustrating a display screen, on which a noninterlace signal is displayed, of a conventional liquid crystal display apparatus. FIG. 9 shows the polarity of a signal voltage applied to each of pixels and also shows an even-numbered frame 111 and an odd-numbered frame 112 by comparison. Further, FIG. 9 illustrates a case where a same image is repeatedly displayed in each of the even-numbered frame 111 and the odd-numbered frame 112 on the screen and where the image shown in the even-numbered frame 111 and the image shown in the odd-numbered frame 112, which differ from each other, are alternately displayed. The even-numbered frame 111 is obtained by performing scanning-line interpolation on an even-numbered field in the interlace signal. The signal voltage applied to each of the pixels in display areas 111b and 111c other than the display area 111a on the screen is set to be equal to a common voltage (that is, a voltage applied to a common electrode). That is, in the display area 111a, a signal voltage, whose polarity is reversed every pixel and every gate line 7, is applied to each of the pixels. In the display areas 111b and 111c, the signal voltage applied to each of the pixels is set to be equal to the common voltage. In the case of the “normally black” type, the display area 111a is displayed white, while the display areas 111b and 111c are displayed black.
Meanwhile, the odd-numbered frame 112 is obtained by performing scanning-line interpolation on an odd-numbered field in the interlace signal. A signal voltage applied to each of the pixels in a display area 112c other than display areas 112a and 112b on the screen is set to be equal to the common voltage. The display areas 112a to 112c are the same as those 111a to 111c, respectively. Under each of the display screen, the signal voltage in the display areas 111b and 112b, which are the boundaries between the white display part and the black display part, are illustrated corresponding to the pixels provided on the gate line 7. In a case where such an even-numbered frame 111 and such an odd-numbered frame 112 are alternately and repeatedly displayed over plural frames, the alternatively current driving cannot be performed on each of the pixels in the display regions 111b and 112b, which are the boundaries between the white part and the black part. That is, in the display areas 111b and 112b, the application of a signal voltage having opposite polarity is not performed for a certain period.
FIG. 10 is a view illustrating the polarity of a signal voltage applied to a pixel every vertical scanning period in the conventional liquid crystal display apparatus. In the display areas 111b and 112b, the polarity of a signal voltage applied to each of the pixels is alternately 0 and, for instance, negative every vertical scanning period. Thus, the application of the signal voltage of the opposite polarity is not performed, so that the direct-current component of the signal voltage is accumulated in each of the pixels. When the direct-current component is accumulated therein, each of the pixels causes image sticking. This has adverse effects. For example, a pixel portion, in which the image sticking occurs, becomes a residual image (or causes flicker). FIG. 11 illustrates a display area 113a, which becomes a residual image after on the display area 113 displayed after the display screen shown in FIG. 9 is displayed (and which is the same area as each of the display areas 111b and 112b).
FIGS. 12 and 13 are state transition views each illustrating the display screen of the conventional liquid crystal display apparatus every frame. As shown in FIG. 12, the polarity of the signal voltage is reversed so that the signal voltage has the opposite polarities thereof alternately between the even-numbered frames 121, 123, and 125 and the odd-numbered frames 122, 124, and 126. Thus, the image sticking of the pixel does not occur. In contrast with this, in a case shown in FIG. 13, display areas other than those 131a, 133a, and 135a in even-numbered frames 131, 133, and 135, and all display areas in odd-numbered frames 132, 134, and 136 have the common voltage. In the display areas 131a, 133a, and 135a, no alternating current driving cannot be performed. Thus, it is possible that in a case where such a video signal is inputted, the direct-current component is accumulated in each of the pixels in the display areas to thereby cause image sticking.
As described above, the conventional liquid crystal display apparatus has problems that when a noninterlace signal obtained by performing scanning-line interpolation on an interlace signal is displayed therein, a pixel, in which alternating-current driving cannot be performed, appears in a case where a same image is continuously displayed over plural frames, and that defective indication, such as image sticking, is caused.